Turbochargers are well known devices for supplying air to an intake of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to an engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housings.
In known turbochargers, the turbine stage comprises a turbine chamber within which the turbine wheel is mounted; an annular inlet passageway defined between facing radial walls arranged around the turbine chamber; an inlet volute arranged around the inlet passageway; and an outlet passageway extending from the turbine chamber. The passageways and chambers communicate such that pressurised exhaust gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine and rotates the turbine wheel. It is also known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passageway so as to deflect gas flowing through the inlet passageway towards the direction of rotation of the turbine wheel.
Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the inlet passageway can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suit varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level which ensures efficient turbine operation by reducing the size of the annular inlet passageway. Turbochargers provided with a variable geometry turbine are referred to as variable geometry turbochargers.
In one known type of variable geometry turbine, an axially moveable wall member, generally referred to as a “nozzle ring”, defines one wall of the inlet passageway. The position of the nozzle ring relative to a facing wall of the inlet passageway is adjustable to control the axial width of the inlet passageway. Thus, for example, as gas flow through the turbine decreases, the inlet passageway width may be decreased to maintain gas velocity and optimise turbine output.
The nozzle ring may be provided with vanes which extend into the inlet and through slots provided in a “shroud” defining the facing wall of the inlet passageway to accommodate movement of the nozzle ring. Alternatively vanes may extend from the fixed facing wall and through slots provided in the nozzle ring.
Typically the nozzle ring may comprise a radially extending wall (defining one wall of the inlet passageway) and radially inner and outer axially extending walls or flanges which extend into an annular cavity behind the radial face of the nozzle ring. The cavity is formed in a part of the turbocharger housing (usually either the turbine housing or the turbocharger bearing housing) and accommodates axial movement of the nozzle ring. The flanges may be sealed with respect to the cavity walls to reduce or prevent leakage flow around the back of the nozzle ring. In one common arrangement the nozzle ring is supported on, or is supported by, rods (sometimes referred to as “pushrods”, “push-rods” or “push rods”) extending parallel to the axis of rotation of the turbine wheel. The nozzle ring is moved by an actuator assembly which axially displaces the rods.
An example of such a known actuator assembly is disclosed in U.S. Pat. No. 5,868,552. A yoke is pivotally supported within the bearing housing and defines two arms, each of which extends into engagement with an end of a respective nozzle ring support rod.
The yoke is mounted on a shaft journaled in the bearing housing and supporting a crank external to the bearing housing which may be connected to an actuator in any appropriate manner. Each arm of the yoke engages an end of a respective support rod via a block which is pivotally mounted to the end of the yoke on a pin and which is received in a slot defined by the rod which restrains the block from movement along the axis of the rod but allows movement perpendicular to the axis of the rod. An actuator is controlled to pivot the yoke about its support shaft via the yoke crank which in turn causes ends of the yoke arms to describe an arc of a circle. Engagement of the yoke arms with the nozzle ring support rods moves the rods back and forth along their axis. Off axis movement of the yoke arms is accommodated by the sliding motion of the blocks within the slots defined by support rods.
The actuator which moves the yoke can take a variety of forms, including pneumatic, hydraulic and electric forms, and can be linked to the yoke in a variety of ways. The actuator will generally adjust the position of the nozzle ring under the control of an engine control unit (ECU) in order to modify the airflow through the turbine to meet performance requirements.
In use, a torque may be imparted onto the nozzle ring due to gas flow in the turbine. This is particularly the case if the nozzle ring is provided with a plurality of vanes arranged, in use, to deflect gas flowing through the inlet passageway of the turbine towards the direction of rotation of the turbine wheel. A torque on the nozzle ring is also applied to the rods which support the nozzle ring. Torque acting on the rods may push a side of the rods against one or more guides which guide movement of the rods (for example, a bush or bushing or the like). On an opposite side of the rod, where no torque is applied, oil may leak along the rod. Due to the high temperature of the turbocharger environment, the oil may coke. The coke may build up, and over a period of time may inhibit or prevent movement of the rod along the guide.
The dimensions of a nozzle ring may depend on, for example, the type of variable geometry turbocharger, or on the properties of the variable geometry turbocharger. For instance, for aerodynamic reasons, it may be desirable to reduce the radial extent of the nozzle ring (i.e. the extent to which the nozzle ring extends in the radial direction, or in other words the distance between the inner radius and outer radius of the nozzle ring). Even though it may be desirable to provide a nozzle ring with a reduced radial extent, it may at the same time be desirable not to have to re-design or manufacture other related parts in a different way in order to take into account the change in dimensions of the nozzle ring. For example, if the radial extent of the nozzle ring is reduced, commonly used rods may no longer fit into a cavity formed by the nozzle ring and into which the rods extend in order to fix the rods to the nozzle ring. It therefore becomes necessary to re-design and manufacture a new rod which can fit into the cavity provided in the nozzle ring. A typical example of such a re-designed rod would be a rod which is smaller, such that the diameter of the rod is reduced along the entire length of the rod. However, the mechanical properties of such a rod may not be adequate for use in supporting the nozzle ring. For instance, the rod may not be sufficiently stiff or robust enough to withstand the forces and temperatures that, in use, the rod would be subjected to in the turbocharger environment.